![]() However, if the ISP has already issued a DHCP lease for the MAC address requesting an IP address, these requests may go unanswered. ![]() Capturing packets on that interface (using a len>0 filter or equivalent) will confirm that the interface is sending DHCP requests. SOL2035P3 SOL2035P3A 3 voice disabled 10.120.135.193/27Īs you can see here, the DIA interface has no IP address. An example is below (output is truncated to fit the show network-interface node all When you see a DHCP-enabled interface that has an value for its address, this is a possible culprit. ![]() Then, without warning, it will suddenly spring to life and acquire an IP address. An interface may suddenly "lose" its IP address, and dutifully send DHCP requests to acquire a new address, but have those requests go unanswered. If MAC addresses are not unique in a domain, this will lead to peculiar behavior as observed by the network admin. For this reason, in the remainder of the document we will refer to the problem as existing in a common "domain," which may represent either a broadcast domain or a DHCP domain. In one instance, a nationwide enterprise branch location shared a common broadcast domain with anoter branch location over 12 miles away, using the same ISP.įurthermore, data suggests that the issue may not necessarily be limited to broadcast domains if an ISP uses shared DHCP servers that cover many broadcast domains, these may be unwilling to issue DHCP leases to two different broadcast domains if the DHCP requests share a common L2 source address. We have observed cases where broadcast domains span much further than anticipated, and reusing the same shared-phys-address on two routers many miles apart has contributed to difficult-to-diagnose issues. Furthermore, for the reasons mentioned above, this shared-phys-address must be otherwise unique on its broadcast domain. Thus, enabling this behavior only requires assigning a unique shared-phys-address that is shared between the two devices. The 128T router uses this technique for its "protected interface" failover by assigning the same shared-phys-address on two interfaces spanning two nodes that comprise a router, you can have those nodes elect an active owner of the interface, and transfer ownership to the partner node in the event of a failure. These addresses must be unique on each L2 (broadcast) domain, in order to ensure that Ethernet frames are sent to the appropriate device.īecause two devices with the same MAC on the same L2 broadcast domain are indistinguishable from one another, this is often leveraged by equipment manufaturers for the purposes of interface failover deliberately deploying two devices with the same MAC on the same broadcast domain, and controlling which of them "owns" the interface lets adjacent devices continue to communicate with the same L2 destination when ownership changes from one piece of equipment to another. Each Ethernet device uses an address referred to as the Media Access Control (MAC) address that allows other devices to communicate with it at the Data Link layer.
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